Aqueous-phase Sulfate Production Research Articles

Aqueous phase oxidation of bisulfite influenced by nitrate and its photolysis

Nitrate aerosol is ubiquitous in the atmosphere. Nitrate in the particulate and aqueous phase can affect various atmospheric chemical processes through its hygroscopicity and photolysis. The impacts of nitrate photolysis on the heterogeneous oxidation of SO2 have been attracting attention. However, the influence of nitrate on heterogeneous aqueous phase formation of atmospheric sulfate aerosol is still not very clear. In this study, the effects of nitrate on aqueous phase oxidation of bisulfite under different conditions were investigated. Results show that nitrate photolysis can promote the oxidation of bisulfite to sulfate, especially in the presence of O2. It is found that pH plays a significant role in the reaction, and ammonium sulfate has significant impacts on the enhancement of aqueous phase sulfate production through regulating the pH of solution. An apparent synergism is found among halogen chemistry, nitrate and its photochemistry and S (IV) aqueous oxidation, especially the oxidation of halide ions by nitrate and its photolysis and by the intermediate products produced by the free radical chain oxidation of S (IV) in acidic solution, leading to the coupling of the redox cycle of halogen with the oxidation of bisulfite, which promotes the continuous aqueous oxidation of bisulfite and the formation of sulfate. In addition, the role of nitrate itself in the aqueous phase oxidation of bisulfite is revealed. These results provide a new insight into the heterogeneous aqueous phase oxidation pathways and mechanisms of SO2 in cloud and fog droplets and haze particles.

Formation of droplet-mode secondary inorganic aerosol dominated the increased PM2.5 during both local and transport haze episodes in Zhengzhou, China

The size distribution and formation of secondary inorganic aerosol play a key role in the increasing PM2.5 concentration. Size-segregated data including mass, number, and chemical component concentrations were obtained during a haze episode from January 12 to 23 in Zhengzhou to gain insight into the dominant factors for the growth of PM2.5. PM2.5 levels during two local processes (LP1 and LP2) were mainly affected by the accumulation and secondary formation of local pollutants. The transport process (TP) was affected by the air mass transported from the northern area of Zhengzhou. Results show that the growth of particle mass concentration in LP1 mainly occurred in the size range of 400–640 nm and 640–1000 nm. With the aggravated particles increases (LP2), 640–1000 nm and 1–1.6 μm particles dominated the increasing PM2.5 concentration. The particles carried by northern air mass (TP) were concentrated in the size range of 1–1.6 μm. Variation trends of hourly PM2.5 chemical components and size distribution of water-soluble inorganic ions suggested that the formation and growth of droplet-mode nitrate, sulfate, and ammonium dominated the increase of PM2.5, and the particle sizes of these components increased with the increasing PM2.5. High concentrations of aerosol water content and large surface area in droplet-mode were beneficial for the heterogeneous reactions for droplet-mode nitrate formation. Moreover, large particle surface area in droplet-mode particles also provided adequate carriers for the adsorption and condensation of gaseous HNO3 onto these particles. Elevated aerosol water, surface area, and particle acidity enhanced the H2O2 and transition metal (TMI) oxidation for aqueous-phase droplet-mode sulfate formation. The contribution of TMI-catalyzed oxidation significantly increased in LP2 because of the high TMI concentration and particle acidity. Relatively low aqueous-phase sulfate production rates in TP suggest that the observed high concentration of droplet-mode sulfate was mainly originated from the completely transformed SO42− carried by air masses. Moreover, droplet-mode particles exhibited moderate acidity, which enhanced the gas-particle partitioning of NH3(g)/NH4+(a).

Evaluating the Response of Summertime Surface Sulfate to Hydroclimate Variations in the Continental United States: Role of Meteorological Inputs in the GEOS‐Chem Model

AbstractUnderstanding the response of sulfate to climate change is crucial given tight couplings between sulfate and the hydrological cycle. As the sources and sinks of sulfate are sensitive to cloud and precipitation processes, the accuracy of model simulations depends on the accuracy of these meteorological inputs. In this study, we evaluate the GEOS‐Chem model in simulating summertime surface sulfate concentrations in the continental United States across different levels of dryness and compare the model performance based on two sets of meteorological fields: Modern Era Retrospective Analysis for Research and Applications (MERRA) and MERRA‐2. Both simulations fail to reproduce observed increases in sulfate during drought, as indicated by negative correlation slopes between surface sulfate concentrations and the standardized precipitation evapotranspiration index (SPEI). This deficiency can be largely attributed to too large a decrease in clouds and hence aqueous phase sulfate production as conditions shift from wet to dry. MERRA‐2‐driven GEOS‐Chem (M2GC) shows improvements in cloud and precipitation fields relative to the MERRA‐driven GEOS‐Chem, hence eliminating approximately half of the bias in the simulated sulfate‐SPEI slope. However, M2GC still underestimates boundary layer cloud fraction, overestimates liquid water content, and overestimates the rates of the decrease in both quantities as conditions become drier. Explicitly correcting these cloud biases in M2GC results in a 60–80% reduction of the bias in the simulated sulfate‐SPEI slope. The strong sensitivity of simulated sulfate to prescribed cloud fields suggests the need for more comprehensive assessment of cloud inputs for sulfate simulations under current and future climate change scenarios.

A comparison of two chemistry and aerosol schemes on the regional scale and the resulting impact on radiative properties and liquid- and ice-phase aerosol–cloud interactions

Abstract. The complexity of atmospheric aerosol causes large uncertainties in its parameterization in atmospheric models. In a process-based comparison of two aerosol and chemistry schemes within the regional atmospheric modeling framework COSMO-ART (Consortium for Small-Scale Modelling, Aersosol and Reactive Trace gases extension), we identify key sensitivities of aerosol parameterizations. We consider the aerosol module MADE (Modal Aerosol Dynamics model for Europe) in combination with full gas-phase chemistry and the aerosol module M7 in combination with a constant-oxidant-field-based sulfur cycle. For a Saharan dust outbreak reaching Europe, modeled aerosol populations are more sensitive to structural differences between the schemes, in particular the consideration of aqueous-phase sulfate production, the selection of aerosol species and modes, and modal composition, than to parametric choices like modal standard deviation and the parameterization of aerosol dynamics. The same observation applies to aerosol optical depth (AOD) and the concentrations of cloud condensation nuclei (CCN). Differences in the concentrations of ice-nucleating particles (INPs) are masked by uncertainties between two ice-nucleation parameterizations and their coupling to the aerosol scheme. Differences in cloud droplet and ice crystal number concentrations are buffered by cloud microphysics as we show in a susceptibility analysis.

Aqueous phase sulfate production in clouds in eastern China

Clouds play an important role in the oxidation of sulfur dioxide to sulfate, since aqueous phase sulfur dioxide oxidation is typically much faster than oxidation in the gas phase. Important aqueous phase oxidants include hydrogen peroxide, ozone and oxygen (catalyzed by trace metals). Because quantities of emitted sulfur dioxide in China are so large, however, it is possible that they exceed the capacity of regional clouds for sulfate production, leading to enhanced long-range transport of emitted SO2 and its oxidation product, sulfate. In order to assess the ability of regional clouds to support aqueous sulfur oxidation, four field campaigns were conducted in 2007 and 2008 at Mt. Tai in eastern China. Single and 2-stage Caltech Active Strand Cloudwater Collectors were used to collect bulk and drop size-resolved cloudwater samples, respectively. Key species that determine aqueous phase sulfur oxidation were analyzed, including cloudwater pH, S(IV), H2O2, Fe, and Mn. Gas phase SO2, O3, and H2O2 were also measured continuously during the campaigns. A wide range of cloud pH values was observed, from 2.6 to 7.6; 60% of cloud samples had a pH between 3 and 5. H2O2 was found to be an important sulfur oxidant, especially at cloudwater pH lower than 5. H2O2 was the most important oxidant in 68% of the cloud sampling periods. High concentrations of residual H2O2 were observed in many periods, especially in summer, implying a substantial capacity for additional sulfur oxidation. O3 was an important oxidant when cloudwater pH was higher than approximately 5–5.3, and was the most important oxidant in 20% of the studied periods. Aqueous sulfur oxidation by O2 (catalyzed by Fe and Mn) was found to be the fastest sulfate production pathway in 12% of the cases. Observed chemical heterogeneity among cloud drop populations was found to enhance rates of S(IV) oxidation by ozone and enhance or slow metal-catalyzed S(IV) autooxidation rates in some periods. These effects were found to be only of minor importance for the total S(IV) oxidation rate, however, as H2O2 was the dominant S(IV) oxidant during most periods.

Estimates of aqueous-phase sulfate production from tandem differential mobility analysis

Abstract During June and July of 2004, airborne measurements of size-resolved aerosol properties were made over the coastal Pacific near Marina, California. Tandem differential mobility analysis was used to determine the hygroscopic properties of these aerosols and, subsequently, to examine the change in soluble mass after the aerosol had been cloud-processed. Three of the eight cases analyzed during the field campaign exhibited increased soluble mass attributable to cloud-processing. The calculated change in soluble mass after cloud-processing, derived from measurements made below and above cloud, ranged from 1.08 ϒ g m−3 to 1.40 ϒ g m−3. These values are in agreement with those determined using data collected during previous field studies in the same region. Aerosol size distributions measured throughout the study with a Passive Cavity Aerosol Spectrometer Probe were averaged to create a single representative distribution, which was used to examine the impact of the addition of sulfate to a typical aerosol measured during this project. Mass light-scattering efficiencies were calculated for both the initial and cloud-processed size distributions using Mie-Lorenz theory. These calculations show that the increase in mass light-scattering efficiency following passage through a single non-precipitating cloud to be 14%, which is in agreement with findings of previous studies in the same region. This new technique produces results that are consistent with existing methods for measuring in-cloud sulfate production and has the advantage of intrinsic consistency with the hygroscopicity and CCN activity of the aerosol.

Enhancement of CCN concentration due to aqueous phase sulfate production in polluted air

Enhancement of CCN concentration due to aqueous phase sulfate production in polluted air

The influence of drop size-dependent fog chemistry on aerosol processing by San Joaquin Valley fogs

Drop size-resolved measurements of fog chemistry in California's San Joaquin Valley during the 1995 Integrated Monitoring Study reveal that fog composition varies with drop size. Small fog drops were less alkaline and typically contained higher major ion (nitrate, sulfate, ammonium) concentrations than large drops. Small drops often contained higher concentrations of Fe and Mn than large drops while H 2O 2 concentrations exhibited no strong drop size dependence. Simulation of an extended fog episode in Fresno, California revealed the capability of a drop size-resolved fog chemistry model to reproduce the measured (based on two drop size categories) drop size dependence of several key species. The model was also able to satisfactorily reproduce measured species-dependent deposition rates (ammonium>sulfate>nitrate) resulting from fog drop sedimentation. Both the model simulation and direct analysis of size-resolved fog composition observations and measured gas-phase oxidant concentrations indicate the importance of ozone as an aqueous-phase S(IV) oxidant in these high pH fogs. Due to the nonlinear dependence of the rate law for the ozone pathway on the hydrogen ion concentration, use of the average fog drop composition can lead to significant underprediction of aqueous phase sulfate production rates in these chemically heterogeneous fogs.

The influence of chemical heterogeneity among cloud drop populations on processing of chemical species in winter clouds

Drop size-resolved measurements of winter cloud composition in the Rocky Mountains of northern Colorado revealed significant variations of cloud drop pH, ion (SO 4 2−, NO 3 −, NH 4 +, and Ca 2+) concentrations, and concentrations of trace metal catalysts (Fe and Mn) with drop size. Simultaneous measurements of snow chemical composition and the degree of cloud drop capture by snow crystals (riming) revealed a positive correlation between snow composition and the extent of ice crystal riming in two of four cases studied. Observations indicate that the size-dependent chemical composition of the clouds tends to enhance aqueous phase sulfate production rates when the primary oxidant is ozone. Enrichment of accumulation mode aerosol species in small cloud drops, which are inefficiently scavenged by ice crystals, appears to cause small (typically less than 15%) reductions in the efficiency with which these species are scavenged by precipitation during accretional ice crystal growth.

Cloud chemistry varies with drop size

Samples of fog and cloud drops were collected by drop size from several locations around the United States. Sites were selected to represent a variety of cloud types (coastal stratus clouds, frontal clouds, and radiation fog) as well as both clean and polluted environments. A size‐fractionating version of the Caltech Active Strand Cloudwater Collector (sf‐CASCC) and a two‐stage Eidgenössische Technische Hochschule (ETH) cloud impactor were each used to simultaneously collect small and large cloud drop fractions. The sf‐CASCC is designed to collect drops >23 μm in diameter in the large fraction and drops between 4 and 23 μm in the small fraction, while the ETH impactor is typically configured to collect drops >10 μm in diameter and drops between 3 and 10 μm in diameter in its two size fractions. Concentrations of sulfate and ammonium were typically observed to be enriched in small cloud drops. A similar pattern was observed for nitrate at several locations. Coarse mode aerosol species were often observed to be enriched in large cloud drops. These findings suggest that the size‐dependent chemical signature of the precursor aerosol particle size distribution is at least partially maintained in the cloud drop size spectrum. The observed size dependence of the cloud drop composition can enhance rates of aqueous phase sulfate production relative to expectations based on average cloud drop composition, bias efficiencies with which individual chemical species are incorporated into precipitation within clouds, and affect patterns of chemical deposition to plant canopies or within the respiratory tract.

The effects of clouds on aerosol and chemical species production and distribution: 2. Chemistry model description and sensitivity analysis

A modeling study of the effects of cumulus convection on the transformation and redistribution of chemical species in the troposphere is described. A sulfate chemistry model is coupled with a two‐dimensional, mixed‐phase, two‐moment microphysics Eulerian cloud model to examine the importance of different pathways for aqueous sulfate production. The complexation of CH2O with S(IV) was found to be of minor importance in most of the model cloud, compared with the oxidation of S(IV) by H2O2 and O3, while Fe(III)‐catalyzed oxidation plays an important role in aqueous phase sulfate production. Significant redistribution of SO2 from the boundary layer to the middle and upper troposphere was predicted, with a larger percentage redistributed in cases initialized with chemical fields representative of polluted continental conditions. The sensitivity of the S(IV) production and SO2 redistribution to variations in several model parameters is also examined.

Chemical consequences of the initial diffusional growth of cloud droplets: a clean marine case

A simple microphysical cloud parcel model and a simple representation of the background marine aerosol are used to predict the concentrations and compositions of droplets of various sizes near cloud base. The aerosol consists of an externally-mixed ammonium bisulfate accumulation mode and a sea-salt coarse particle mode. The difference in diffusional growth rates between the small and large droplets as well as the differences in composition between the two aerosol modes result in substantial differences in solute concentration and composition with size of droplets in the parcel. The chemistry of individual droplets is not, in general, representative of the “bulk” (volume-weighted mean) cloud water sample. These differences, calculated to occur early in the parcel's lifetime, should have important consequences for chemical reactions such as aqueous phase sulfate production. DOI: 10.1111/j.1600-0889.1989.tb00124.x